Kirei is the Master of Assassin. Before the Fourth Holy Grail War, Kirei served as a priest under his father, Risei Kotomine. Although Kirei understood the distinction between good and evil, he could only revel in committing sins. Hounded by guilt and sorrow and with no alternative, Kirei prayed that, through marriage to a woman he loved, he could finally achieve respite from his tortured existence.